This invention relates to an apparatus and method utilizing time domain reflectometry for measuring the condition or characteristics of a material. Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advantages in micropower impulse radar (MIR), also known as ultra-wide band (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and low cost time domain reflectometry (TDR) devices. Existing TDR devices are expensive and often impractical for industrial level instrumentation.
In a TDR instrument, a very fast stream of pulses with a rise time of 500 picoseconds, or less, is propagated down a transmission line that serves as a probe in a vessel. The pulses are reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. Alternatively, the transition could be two different liquids. The amplitude of the reflected signal depends on the difference between the dielectrics of the two media. The dielectric of air is one, while the dielectric of water is about eighty. The larger the difference in dielectric, the larger the reflected signal.
McEwan, U.S. Pat. No. 5,345,471, and other related patents, describe a technique to transmit and receive very fast pulses with simple, low cost and low power electronics. More particularly, McEwan, U.S. Pat. No. 5,609,059, describes a level sensor utilizing this technology. However, the device described therein is intended to be used for simple commercial level applications, such as automobile engine fluid levels. It does not utilize the feature set, power consumption, and versatility required for use in the industrial process environment. Other known devices utilize this technology for a two-wire transmitter using just two wires for both receiving power from the user and sending level information to the user. However, these devices are analog devices limited in the ability to measure the level of extremely low dielectric materials, or materials that coat, clump or build up on the probe, over the wide temperature extremes of industrial process level environments. They also have limited level range capability.
Guided wave radar is one of many techniques available to measure the level of liquids or solids in an industrial environment. Guided wave radar works by generating a stream of pulses of electromagnetic energy and propagating the pulses down a transmission line formed into a level sensing probe. The probe is generally placed vertically in a tank or other container and the electromagnetic pulse is launched downward from the top of the probe. The probe is open to both the air and the material to be sensed in such a way that the electromagnetic fields of the propagating pulse penetrate the air until they reach the level of the material. At that point, the electromagnetic fields see the higher dielectric of the material. This higher dielectric causes a reduction in the impedance of the transmission line, resulting in a pulse echo being reflected back to the top of the probe. The pulse travels through the air dielectric portion of the probe at a known velocity. This allows the material level on the probe to be determined by measuring the round trip travel time of the pulse from the top of the probe to the level and back to the top of the probe. Conductive materials generate echoes similar to the echoes from high dielectric materials. Therefore, the same measurement technique also works with conductive materials.
The probes are often constructed in the form of coaxial or twin conductor transmission lines. However, a probe in the form of a single conductor transmission line has a number of advantages in certain cases. An important property of the single conductor probe is that its electromagnetic fields extend much further from the probe than do the electromagnetic fields in either a coaxial or twin conductor probe. Therefore, the single conductor probe responds to the effective dielectric constant seen over a significant volume surrounding the probe. This allows the probe to respond to a material level but not respond to materials that coat the probe or stick to the probe in clumps. A second important property of a single conductor probe is that it is inherently a high impedance device compared to other transmission line types. Therefore, there is an unavoidable large impedance rise at the top of the probe where the transmission line changes from a coaxial, two-conductor, or other type of feed line to the single conductor transmission line. This impedance rise reflects a large fraction of the transmitted energy back to the receiver. Only a relatively small amount of the transmitted energy is propagated down the probe and is available for level sensing. Therefore, pulses reflected from a material level will be much smaller than the echoes from the top of probe impedance rise. Material levels sufficiently close to the top of the probe will not be sensed because the large reflected pulse from the top of the probe will overlap and obscure the smaller reflected pulse of opposite polarity from the material level.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.
In accordance with the invention there is provided a TDR measurement instrument including improved calibration and measurement features.
Broadly, there is disclosed herein in accordance with one aspect of the invention, a time domain reflectometry measurement instrument comprising a probe defining a transmission line. A pulse circuit is connected to the probe for generating pulses on the transmission line and receiving reflected pulses returned on the transmission line, the reflected pulses representing a characteristic of a material being measured. A measuring circuit is connected to the pulse circuit for developing a representation of the reflected pulses. The measuring circuit is adapted to automatically adjust sampling responsive to variation in dielectric constant of material being traversed by the transmission line.
Further features and advantages of the invention will be readily apparent from the specification and from the drawing.